Gold ores are treated by a variety of processes. All hydrometallurgical gold recovery processes rely on leaching relatively low concentrations of gold from ores using lixiviants, primarily cyanide solutions. Ores that contain gold extracted by comminuting and followed by leaching with cyanide solution are known as “oxide” or “free milling” ores. However, gold recovery from most ores by cyanide leaching is not effective, with as little as 30 percent, or even lower, of the gold content of the ore being amenable to cyanide leaching. These ores are commonly termed “refractory” ores. Poor gold recovery from refractory ores is typically caused by the gold being occluded in sulfide mineral grains (usually arsenopyrite and/or pyrite grains) so that the gold cannot react with the cyanide leach solution or by dissolved gold being adsorbed by carbonaceous material present in the ore (this phenomenon is known as “preg robbing”) Ores having both types of metallurgical problems (occluded and preg robbing) are commonly known as “double refractory” ores. Further losses in gold recovery can occur when dissolved gold is occluded by inorganic precipitates which typically occur during autoclave treatment of refractory gold ores.
A common method of treating refractory gold ores is by pressure oxidation in autoclaves. Pressure oxidation oxidizes sulfide minerals, rendering the residue non-refractory. The gold is then dissolved by cyanidation and concentrated by adsorption onto activated carbon (either in adsorption columns or in carbon added to the leaching process (known as Carbon-In-Leach (“CIL”) or Carbon-In-Pulp (“CIP”) techniques) or onto a resin (known as the Resin-In-Pulp (“RIP”) technique). The adsorbed gold is eluted from the adsorbed carbon by washing and stripping with ammonia, nitric acid, hydrochloric acid, caustic solution, and/or steam. The gold is then converted to a solid from the eluate by electrowinning (electroplating of gold onto cathodes), precipitation and filtration, or cementation.
Prior to precious metal recovery, the autoclave discharge is either directly neutralized after cooling or subjected to a solid/liquid separation to remove acid and dissolved metals. If either cyanidation or thiosulphate is employed, the pH of the pulp must be increased to an alkaline pH to avoid the formation of hydrogen cyanide or cause thiosulphate destruction.
Pressure oxidation converts sulfide sulfur in minerals such as pyrite FeS2 and arsenopyrite FeAsS, into sulfate sulfur. Small amounts of iron and arsenic in the sulfide materials are also converted to the dissolved ferrous iron, ferric iron, arsenite and arsenate. Under these conditions, iron is precipitated in the autoclave as goethite, hematite (Fe2O3) and scorodite (FeAsO4.2H2O), and sulfuric acid is generated in solution. These two iron compounds are very desirable because they are chemically stable. It is possible to form other stable Fe—As compounds in the autoclave, depending on the temperature, the Fe/As ratio, and the acidity in the autoclave liquor. Because of their chemical stability, these compounds are inert during the subsequent neutralization and cyanidation steps and, therefore, do not consume expensive chemicals, such as lime.
Depending on the chemical conditions prevailing in the autoclave, other less desirable iron compounds can be formed. Examples of such compounds include basic iron sulphate, FeOHSO4, and jarosite, X Fe3(SO4)2(OH)6, where X is typically one of H3O+, Na+, K+, NH430, ½Pb2+, and Ag+.
Jarosites and basic iron sulphates can be chemically instable. For example, in the autoclave discharge, basic iron sulphate can react with lime during pre-cyanidation neutralization to form ferric hydroxide and calcium sulphate:FeOHSO4+Ca(OH)2+2H2O=Fe(OH)3+CaSO4.2H2O  (1)Also, some jarosites, particularly hydronium jarosite, react with lime during pre-cyanidation neutralization, to form ferric hydroxide and calcium sulphate:(H3O)Fe3(SO4)2(OH)6+2H2O+2Ca(OH)2→3Fe(OH)3+2CaSO4.2H2O  (2)
The presence and relative quantities of hematite, basic ferric sulphate, ferric arsenate and various forms of jarosite can have a major impact on the method and economics of subsequent processes, and largely depends upon the nature of the starting material and the acidic pressure oxidation leach conditions. Generally, pressure oxidation under high acid conditions favours basic iron sulphate and possibly jarosite formation while low acid conditions favour hematite formation. When pressure oxidation is operated under conditions which favour hematite formation, the feed's sulfide sulphur content is converted to free sulphuric acid and dissolved metal sulphates in the solution phase (such as dissolved ferric sulphate), and, if calcium is present, as chemically stable and inert calcium sulphate in the solid phase. Neutralization of the free acid and dissolved sulphate salts in this type of autoclave discharge can be achieved inexpensively with limestone (CaCO3), which is usually a very cost-effective reagent. In some circumstances, where access to, or availability of a suitable limestone deposit, is not possible, even the cost of limestone can be prohibitively expensive. When the autoclave is operated under conditions that favour the formation of residues rich in basic iron sulphate and jarosite, it can have a significant negative economic impact on subsequent precious metal recovery operations. Adequate neutralization of basic iron sulphate and/or jarosite can be accomplished only with stronger and more expensive neutralization agents, such as lime, CaO, or sodium hydroxide, NaOH.
U.S. Patent Application 2006/0133974, published Jun. 22, 2006, and entitled “Reduction of Lime Consumption When Treating Refractory Gold Ores or Concentrates” teaches the use of a hot curing process, as an effective method, prior to gold leaching, for reducing the cost of neutralizing acid residues from pressure oxidation. In this process, basic iron sulphate and free sulphuric acid, both contained in the autoclave discharge, react to form dissolved ferric sulphate according to the following equation:2FeOHSO4+H2SO4→Fe2(SO4)3+2H2O  3)This hot curing process has a residence time of 1 to 24 hours and a preferred temperature range of 85° C. to 95° C. Because the ferric sulphate-containing solution can be separated by solid/liquid separation techniques from the precious metal-containing residue, allowing time for basic iron sulphate to convert to dissolved ferric sulphate can reduce the consumption of expensive lime in the neutralization reaction of cyanidation feed in favor of inexpensive limestone. A further benefit of allowing time for the various components of the autoclave discharge to react with one another is that the strong ferric sulphate solution produced can be recovered and recycled to pre-treat the feed to the autoclave. Ferric ions in the recycled solution react with and oxidize sulfides in the autoclave feed material, thereby reducing the requirement and associated expense of oxygen in the autoclave process. In addition, any remaining acid in the recycle solution will react with carbonate minerals, when present in the autoclave feed material, and reduce the subsequent formation of carbon dioxide inside the autoclave and further improve the utilization of oxygen.
While the hot curing process is well suited to the treatment of pressure oxidation residues containing gold and can, relative to conventional pressure oxidation processes, reduce the costs of neutralizing pressure oxidation residues, there remains a need to realize further reduction in residue neutralization costs. Lime consumption remains a major contributor to these costs.